This invention relate to expansion and pre-expansion of expandable polymers, such as expandable polystyrene (EPS) and expandable polyethylene.
As is well known, pre-expansion (partial expansion) of EPS beads is a preliminary step in the manufacture of various articles, such as foam coffee cups, insulating coolers, shape-molded packing for electronic products and the like. For this purpose, pre-expanded polymer beads are introduced into a mold through a filling valve. Within the the mold, the loosely packed beads are caused to expand further until they largely fill the space between the beads and mold surface as well as the spaces between the beads, thereby forming the manufactured article, which can be removed from the mold after cooling. Such processing is disclosed in prior art publications, such as U.S. Pat. No. 3,897,899 issued to Schuffet al on Aug. 5, 1975.
For over 25 years, pre-expansion of EPS beads has been carried out by using steam to provide the thermal energy to soften the unexpanded EPS crystal beads which contain a blowing agent such as pentane. Because steam is an efficient carrier of calories of thermal energy per unit mass, when intermixed with the crystal beads, it causes desirably rapid expansion thereof into much larger pre-expanded beads, each of which comprises a number of hollow cells. Each cell is formed of polymer cell walls, containing somewhat expanded pentane as well as water droplets from condensed steam. After a typically brief cooling period of less than an hour (to set the beads in their pre-expanded state) and a suitable xe2x80x9cconditioningxe2x80x9d period, the pre-expanded beads are next introduced into a mold for formation of a finished article. Conditioning also allows some of the pentane and condensed water vapour to escape through the cell walls to the surrounding atmosphere.
The purpose of the conditioning period (typically 8 to 24 hours) is both to allow the internal pentane pressure within cells and the atmospheric pressure to reach a steady state equilibrium (with concomitant increase in density) as well as to dry the pre-expanded beads sufficiently that condensed water vapour on the surfaces of these beads no longer agglomerates them into lumps that may not easily pass through the filling valve used in filling the mold or may not flow into corners and narrow spaces of the mold itself. Moreover, this conditioning period advantageously permits some of the water droplets (from condensed steam), that are inside the cells to escape through the cell walls thus drying the insides of the beads. Without inside drying, the trapped water droplets sometimes induce local non-uniformities in the molding of articles (e.g. holes in coffee cups) because each droplet requires longer heating to vapourize it before heating and expansion of the surrounding cell can progress. However, care must be taken that the conditioning period is not too long, as too much of the remaining blowing agent (e.g. pentane) may be lost by diffusion out of the cells of the pre-expanded beads, resulting in pre-expanded EPS beads that no longer have the ability to expand further when they are heated during molding. When the beads do not expand sufficiently during molding, the molded products tend to be poorly fused, and often crumble into pieces or, in the case of coffee cups, leak their contents. Thus, for many years, proper conditioning of pre-expanded beads has been a delicate balance between a sufficiently long time needed to dry the condensed steam introduced during pre-expansion, and a sufficiently short time to retain an adequate amount of blowing agent (e.g. pentane) within the pre-expanded beads.
It is now believed that the steam pre-expansion method functions by having the steam penetrate the crystal beads to carry a large thermal effect rapidly into the interior of each bead, where the steam condenses into water vapour, giving up most of its thermal energy by this change of state (at about 540 calories per gram of steam). In other words, it is the penetration of steam into the bead that allows efficient pre-expansion and formation of hollow cells containing thus-expanded pentane as well as water droplets from condensed steam, but this also causes the penetrated steam to condense into water vapor that is now deeply lodged within the cellular structure of each pre-expanded bead. Moreover, the interior cells near the center of each bead will be expanded by the thermal action of penetrated steam upon interior inclusions of blowing agent (e.g. pentane), thereby creating cells containing pentane which have thin interior cell walls that allow greater loss of pentane. This, in turn, gives rise to the delicate balancing required to maintain a conditioning period of the correct length.
In view of the very long history of using steam to provide heat to rapidly pre-expand EPS beads, it is a surprising, and somewhat contrarian, proposal of the present invention to use a less thermally efficient transfer medium in the form of a dry heated gas, such as air, to more slowly pre-expand the crystal polymer beads. It is believed that the use of hot, dry (low water moisture content) air as a media for transferring heat to the crystal beads causes the beads to heat up from the surface towards the center. Due to this slower action by the heated air, which does not readily carry calories into the interior cell structure of the crystal beads (as compared to steam, which provides most of its calories by change of state, rather than conduction), a conductive type of heating of the bead takes place. Dry heated air, which provides no latent heat of condensation (unlike the 540 calories per gram provided by steam), only transfers about 0.24 calories per gram per degree Celsius of temperature difference between the EPS bead and the heated air. It is believed that the outer layer of each bead is first heated by the hot air and that layer by layer the heat penetrates conductively inwardly (both by infrared radiation and by permeation of dry heated air inward from peripheral toward interior cells, together hereinafter sometimes called xe2x80x9cconductive heatingxe2x80x9d), thereby forming a more pre-expanded structure (with thinner cell walls) on the peripheral (i.e. outer) surface of each bead, and a less expanded structure (with thicker cell walls) at the interior (e.g. center) of each bead. As a result, the pre-expanded beads produced by air heating, according to the present invention, contain a higher percent content of blowing agent (e.g. pentane) since it is locked within the interior cells of each bead by their surrounding thick walls of material which have not yet been heated by slow thermal conduction as much as the thin-walled peripheral (outer) cells. The resulting pre-expanded beads are advantageously very dry and can be used immediately in molding of containers and other EPS foam articles due to excellent flow characteristics and excellent expansion capability (due to high pentane content). In other words, the invention permits one to optionally dispense with the conditioning step in the preparation of EPS material for molding of foam articles.
Accordingly, the present invention broadly provides a method of expanding beads of expandable polymer, each bead comprising a mixture of polymer and a blowing agent. This novel method comprises the steps of:
a) heating a quantity of dry gas,
b) maintaining the heated dry gas in contact with said bead until said bead is enlarged to a desired degree due to thermal expansion (primarily by conductive heating and without substantial condensation of the heated dry gas) of said blowing agent therein to form a plurality of both peripheral and interior, mutually joined, cells, each cell comprising said polymer and a quantity of blowing agent therein, and
c) cooling the enlarged beads (as by terminating further contact thereof with the heated dry gas) to set their cells in an enlarged state thereof.
According to a preferred embodiment of the invention, the expandable polymer comprise expandable polystyrene, the gas comprises air, and the blowing agent comprises pentane.
Preferably, due the conductive nature of the heat applied by the heating gas, the aforesaid peripheral cells substantially surround and are substantially larger than said interior cells.
According to a preferred embodiment, heating step b) is carried out by:
i) circulating heated air at the bottom of a chamber, and
ii) then introducing a batch quantity of said beads into said chamber while circulating said heated air therein to stir said beads.
In order to mold articles (e.g. foam coffee cups, forms for concrete, insulated coolers, from beads of expandable polymer (e.g. polystyrene, polyethylene, and the like), where each bead comprises a mixture of the polymer and a blowing agent (e.g. pentane, butane), the present invention provides a method comprising the steps of;
a) heating a quantity of dry gas (e.g. air having a low water moisture content),
b) maintaining the aforesaid heated dry gas in contact with the aforesaid bead until the bead is partially enlarged to a desired degree due to thermal expansion (primarily by conductive heating and without substantial condensation of the heated dry gas) of the aforesaid blowing agent therein to form a plurality of both peripheral and interior, mutually joined, cells, each cell comprising the aforesaid polymer and a quantity of blowing agent therein, by:
i) circulating heated dry gas at the bottom of a chamber, and
ii) then introducing a batch quantity of the aforesaid partially enlarged beads into the aforesaid chamber while circulating the heated gas therein to stir the beads, and
c) cooling the beads (as by terminating further contact thereof with the heated dry gas) to set cells thereof in an enlarged state thereof to thereby form pre-expanded beads,
d) introducing said pre-expanded beads into a mold defining a shape of an article, and
e) heating the aforesaid pre-expanded beads in the afore said mold to further expand said pre-expanded beads to form the desired article.
Preferably, the aforesaid peripheral cells of the beads, that are introduced into the mold, substantially surround and are substantially larger than the aforesaid interior cells.
As a novel product, the present invention provides a pre-expanded bead of expandable polymer (e.g. polystyrene, polyethylene or the like) for molding articles therefrom, the aforesaid pre-expanded bead comprises a plurality of both peripheral and interior, mutually joined, cells, each cell comprising a cell wall of the polymer and a quantity of blowing agent (pentane, butane or the like) enclosed thereby, wherein the peripheral cells of the bead substantially surround and are substantially larger than the interior cells thereof. Preferably, the cell walls of interior cells are generally thicker than the cell walls of exterior cells.
The invention also provides an apparatus for expanding beads of expandable polymer (e.g. Polystyrene, polyethylene, or the like). The aforesaid apparatus comprises:
a) a vertical expansion chamber for receiving a selected batch quantity of the aforesaid beads, the aforesaid expansion chamber comprising a lower portion and an upper portion, the aforesaid lower portion being formed of gas-impermeable material, the aforesaid upper portion being formed of gas-permeable material,
b) a loading bin for discharging the aforesaid selected batch quantity of beads into the aforesaid lower portion,
c) a source of dry heated, pressurized gas (e.g. dry air) connected in gas-communicating relationship with the aforesaid lower portion, the aforesaid source being operable to direct the heated gas into the aforesaid lower portion of the chamber to stir and percolate through the aforesaid selected batch quantity of beads therein to cause heating thereof.
Preferably, the aforesaid source of heated dry air or other dry gas comprises a hot air blower having an output nozzle directed away from a vertical central axis of the aforesaid expansion chamber in order to cause more vigorous stirring of the beads. Of course, the hot air blower may draw air across a heat exchanger or the like before directing it into the lower portion of the chamber. According to a preferred embodiment of the novel pre-expansion apparatus, it may further comprise a sensor operable to detect a selected level of expanded beads within said expansion chamber as well as a discharge outlet in the aforesaid lower portion of the expansion chamber, the discharge outlet being openable to discharge expanded beads upon detection of the aforesaid selected level by said sensor.